#ifdef GL_ES
precision mediump float;
#endif

uniform vec2 u_resolution;
uniform vec2 u_mouse;
uniform float u_time;
//  Function from Iñigo Quiles
//  https://www.shadertoy.com/view/MsS3Wc
vec3 hsb2rgb( in vec3 c ){
    vec3 rgb = clamp(abs(mod(c.x*6.0+vec3(0.0,4.0,2.0),
                             6.0)-3.0)-1.0,
                     0.0,
                     1.0 );
    rgb = rgb*rgb*(3.0-2.0*rgb);
    return c.z * mix(vec3(1.0), rgb, c.y);
}

void main() {
    float steps = 10.0;
    vec2 st = gl_FragCoord.xy/u_resolution.xy;
    float dist = distance(vec2(0.5), st);
    float space = mod(dist * 20.0 - u_time * 2.0, steps);
    vec3 color = hsb2rgb(vec3(ceil(space) / steps, 1.0, fract(space)));

    gl_FragColor = vec4(color, 1.0);
}